Formation of coatings on germanium bodies



United States Patent 3,401,054 FORMATION OF COATINGS 0N GERMANIUM BODIESJohn George Wilkes, Hatch End, Middlesex, England, as-

signor to The General Electric Company Limited, London, England NoDrawing. Filed Sept. 3, 1965, Ser. No. 485,078

5 Claims. (Cl. 117-201) a germanium body a coating of germanium dioxidehaving a tetragonal crystal structure, the body is heated above 400 C.while exposed to an oxidising atmosphere containing water vapour, thetotal pressure of said atmosphere being at least kgs./cm.

The results obtained, in respect of the thickness and quality of theresultant coating, when using such a method depend on a number offactors. Firstly, it may be observed that the best results in respect ofcoherence of the coating are obtained when using relatively hightemperatures (say above 600 C.); with such temperatures, the

results are not very dependent on the total pressure of the oxidisingatmosphere provided this exceeds the minimum indicated above. Theresults are, however, very dependent on the amount of water vapourpresent in the atmosphere; other things being equal, an increase in theamount of water vapour results in an increase in the thickness of thecoating and also causes the coating to have a more granular nature. Thethickness of the coating is also dependent on the time and temperatureof the heat ing, increasing with an increase of either of these factors,while an increase in the time of heating also results in an improvementin the coherence of the coating.

It will be appreciated that when using a method according to theinvention in the manufacture of semiconductor devices, it will normallybe required to provide a good coherent coating of well controlledthickness. To this end, it will normally be desirable to carry out themethod at a temperature above 600 C., and to choose the water vapourcontent of the oxidising atmosphere so as to obtain a coating of thedesired thickness in a time which is sufiiciently short to be convenientfrom the point of view of a manufacturing process but at the same timesufficiently long to ensure that the coating has the required degree ofcoherence; it may be noted that a time of about 20 hours will normallybe suitable having regard to these considerations.

It will normally be convenient to use an atmosphere consistingessentially of oxygen and water vapour only, although in some casesthese active constituents of the atmosphere may be diluted with an inertgas such as nitrogen.

One method of performing the invention will now be described by way ofexample.

In this method a germanium body, which has been chemically cleaned inconventional manner, is placed in a steel high pressure vessel which hasan internal volume of about 100 cc. and which is lined with platinum toprevent contamination of the contents. A controlled amount of water,having a volume in the range 0.1-0.5 cc., is introduced into the vessel,and the vessel is then filled with oxygen at a pressure in the rangel00-150 kgs./cm.

and sealed. The sealed vessel is heated in a suitable furnace to atemperature of about 700 C., being maintained at this temperature forabout 20 hours and then allowed to cool to room temperature. During theheating the oxygen pressure within the vessel increases to about threetimes its initial value; the amount of water introduced into the vesselis such that the partial pressure of water vapour within the vessel issmall compared with that of the oxygen.

As a result, the germanium body has formed on it a coherent adherentcoating of germanium dioxide having a tetragonal crystal structure; thethickness of the oxide coating may be varied over a range ofapproximately one to twenty microns by choice of the precise value,within the range indicated above, of the volume of water intro ducedinto the vessel. The oxide has crystal lattice unit cell dimensions,determined by X-ray analysis, as follows:

that is it is of the form denoted T(I) in British patent specificationNo. 976,559.

I claim:

1. A method of manufacturing a germanium body having formed on itssurface a coherent adherent coating of germanium dioxide having atetragonal crystal structure, which includes the steps of exposing agermanium body, initially free from any surface coating, to an oxidizingatmosphere containing water vapor, the total pressure of said atmospherebeing at least 30 kilograms per square centimeter, and heating the saidbody at a temperature above 400 C., while the body is exposed to saidatmosphere.

2. A method according to claim 1 wherein the said atmosphere consistsessentially of oxygen and water vapor, the partial pressure of the watervapor being small compared with that of the oxygen.

3. A method according to claim 1 wherein the germanium body is heated,while exposed to the said atmosphere, at a temperature above 600 C.

4. A method according to claim 3 wherein the said heating of thegermanium body is continued for about 20 hours.

5. A method of manufacturing a germanium body having formed onits'surface a coherent adherent coating of germanium dioxide having atetragonal crystal structure, which includes the steps of placing agermanium body in a steel pressure vessel lined with platinum, thenintroducing into the said vessel a volume of water in the range of 0.1%to 0.5% of the internal volume of the vessel, then filling the vesselwith oxygen at a pressure in the range of to kilograms per squarecentimeter and sealing the vessel, then heating the sealed vessel to atem perature of 700 C. and maintaining it at this temperature for about20 hours, and then allowing the vessel to cool to room temperaturebefore withdrawing the coated germanium body therefrom.

References Cited UNITED STATES PATENTS 3,298,875 l/1967 Schink 117--201X 3,340,163 9/1967 Bradshaw et al. 117-201 X 3,342,619 9/1967 Chu117-401 FOREIGN PATENTS 219,126 12/ 1958 Australia.

WILLIAM L. JARVIS, Primary Examiner.

1. A METHOD OF MANUFACTURING A GERMANIUM BODY HAVING FORMED ON ITSSURFACE A COHERENT ADHERENT COATING OF GERMANIUM DIOXIDE HAVING ATETRAGONAL CRYSTALS STRUCTURE, WHICH INCLUDES THE STEPS OF EXPOSING AGERMANIUM BODY, INITIALLY FREE FROM ANY SURFACE COATING, TO AN OXIDIZINGATMOSPHERE CONTAINING WATER VAPOR, THE TOTAL PRESSURE OF SAID ATMOSPHEREBEING AT LEAST 30 KILOGRAMS PER SQUARE CENTIMETER, AND HEATING THE SAIDBODY AT A TEMPERATURE ABOVE 400*C., WHILE THE BODY IS EXPOSED TO SAIDATMOSPHERE.